Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen wide use in aerospace applications. Batteries of this type are rechargeable and have an extremely long cycle life and provide a high uniform output during the entire discharge cycle.
A nickel oxide-hydrogen battery has a further advantage in that the pressure in the battery can be used as a quantitative indication of the state of charge. In addition, batteries of this type have an inherent overcharge protection.
In the typical nickel oxide-hydrogen battery, the positive electrodes are generally in the form of flat porous sintered nickel plaques, impregnated with nickel hydroxide, while the negative electrodes are formed from a thin fine mesh nickel screen having a catalyst such as platinum, bonded to one surface of the screen through a hydrophobic polymeric material, such as polytetrafluoroethylene. On discharge of the battery, hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates and becomes dissociated by the catalyst to the monatomic form. The monatomic hydrogen is ionized and combined with hydroxyl ions to form water with an electron being released in the process of forming each hydrogen ion.
In addition, hydroxyl ions are formed at the positive electrode by the reaction of water with the available oxygen component of the nickel oxide. As a result of these reactions, an electron current is produced in the exterior circuit.
On recharging the reaction is reversed, with the recharging being characterized by the regeneration of hydrogen gas at the negative electrode and the reoxidation of the nickel hydroxide as the positive electrode.
Due to the gas pressures involved, the nickel oxide-hydrogen battery is contained within an outer pressure vessel.
The typical metal oxide-hydrogen battery consists of a number of cell modules, each cell module preferably composed of a pair of back-to-back positive electrodes spaced apart by a separator layer, along with a negative electrode disposed adjacent and separated from each positive electrode. Typically a group of cell modules are contained within a single vessel and installations of this type are commonly referred to as "CPV" (common pressure vessel). In a conventional CPV installation a group of cell modules, which are generally rectangular in configuration, are mounted within a cylindrical pressure vessel with the cell modules extending transversely of the axis of the vessel. With this construction there is a substantial gap or space between the sides of the cell modules and the inner wall of the vessel. With a nickel oxide-hydrogen battery, heat is generated in both the charging and discharging cycles and it is important that heat be transferred to the outer vessel and hence to the surrounding atmosphere. Due to the spacing of the rectangular cell modules from the cylindrical vessel, heat transferred to the vessel is minimal with the result that the performance of the battery can be adversely affected.
U.S. Pat. No. 4,957,830 is directed to an improved metal oxide-hydrogen battery in which each cell module is semi-circular in planar configuration and the cell modules are stacked in side-by-side relation to provide a semi-cylindrical stack with the curved peripheral edges of the stack disposed adjacent the inner surface of the vessel, while the straight or chordwise ends of the cell modules are sealed and disposed in opposed relation to the sealed ends of a second stack of cell modules. In addition, heat transfer sheets formed of a thermally conductive material can be positioned flatwise between adjacent cell modules with the outer peripheral edge of the sheet in contact with the inner surface of the vessel to improve heat transfer to the pressure vessel.
While the construction shown in U.S. Pat. No. 4,957,830 substantially improves heat transfer to the outer pressure vessel, the construction requires that the semi-circular positive and negative electrodes be cut from strip stock with the result that there is substantial scrappage of the expensive electrode material which increases the overall cost of the battery.